Sounds like you are making some terrific progress.
I'm going to paste your post in here and make all the m/s to knots conversions as windsurfers normally
speak of windspeeds in kinots on this forum.
"Totally I spent 50-60 hours on water. I sail mostly in fresh water (big inland reservoir app. 12*75 km), usual winds here are 5-8 m/s (9.7-15.5 knots), sometimes up to 15 m/s (29 knots)(but it's unlikely that I will sail in winds more than 10-12 m/s (19.4-23.3 knots) in near future), wind is quite gusty. 5-10 times in a season I may go to seashore for several days looking for more stable winds of 7-12 m/s (13.6-23.3 knots).
Many guys in our club say that I need to switch to a smaller board – most common opinion is 125-135L, some say I can take 115L.
I have tried 125L (69 cm width) for couple of hours (in salt water) - the wind was very light like 3-4m/s
(5.8-7.8 knots)and it was quite choppy. I could uphaul and go, but couldn’t turn.
Last week I took Starboard S-Type 115L ( 64 cm width) form friend of mine and had 6 sessions of 2-3 hours with it (with small rig of 5.0), winds 2-7m/s (3.8-13.6 knots), flat fresh water – I could uphaul and go, but I couldn’t turn (just several tacks during the whole week, and hundreds of uphauls – it was absolutely exhausting and no fun at all)
I had a 2-hour session on 135L (70 cm width) , flat water, 4-6 m/s(7.7 -11.6 knots), 5.0 rig – it was doable, I was quite comfortable doing tacks, the feeling was very similar the first times on a big board.
I had an hour session with 145L(72 cm width) and 7.0.rig, 3-5m/s, flat water – it was quite easy, I did tacks and went upwind.
So the minimum doable volume for me is 135 in flat water, but I'm afraid in choppy conditions it may appear very difficult to sail on 135L."
OK, now that I have a much better idea of the windspeeds, I see you sailing a 5.0 m2 rig in very little wind. This could be a problem, it's nearly impossible to sail if the board will barely move.
In < 8 knots of wind ( 4 m/s) almost no one sails..... it's considered longboard (with a centerboard and a big => 7.5 m2 rig) sailing conditions.
At your skill level, with your 7.0 m2 Pilot rig, a board that's at least 75 cm wide would seem to be what you need.
I might suggest a Starboard GO (140-170 liters).
You are on fresh water most of the time, so that's also a consideration... fresh water has less buoyancy so you need about 5-10 more liters of volume than on salt water.
I think board width may be more of a criteria for you than the volume.
The S-Type 115 is a lovely board, but it's pretty narrow in the tail.
I would guess that you need 130-150 liters, but at least 68-70 cm wide.
Ultimaltely you will want a smaller/narrrower board for wind conditions > 18-20 knots (9.2 m/s - 10.3 m/s)
and a smaller rig ( perhaps 5.5-6.0 m2).
Until you have the balance and skills on a wider board and larger rig in light wind conditions and you do not fall in the water on your transitions, it will be wise to stay with the wider board (the plane much earlier) and larger rigs.
Hope this helps,